Cis- and trans- compounds are geometric isomers. The phenomenon of geometric isomerism is a general one and can be encountered in any class of compounds that contain carbon-carbon double bonds. The cis- differ from trans- only in the way the atoms are oriented in space. One example of a geometric isomers are cis-2-butene and trans-2-butene. Both cis- and trans-2-butene contain one carbon-carbon double bond. The only difference is the orientation of each methyl group on the two carbon-carbon double bond. The two methyl groups will be on the same side of the double bond for cis-2-butene; they will be on the opposite sides for trans-2-butene.
It is known to the art that the separation of geometric isomers can be carried out by either crystallization, precipitation, fractional distillation, solvent extraction and adsorption. Adsorption technique is getting more attention recently because high purity and high efficiency can be obtained from the process. High purity separation of geometric isomers by fractionation is often difficult because of the closeness of the boiling point of the particular isomers, e.g., trans-2-butene boils at 1.degree. C. and cis-2-butene at 4.degree. C.
It is also well known to use aluminosilicates (zeolites) as the adsorbents in the above mentioned adsorption technique. U.S. Pat. No. 2,850,549 to Ray teaches the use of chabazite as an adsorbent in a method for the separation of the components of a pair of cis- and trans-isomers containing the same number of carbon atoms per molecule such as cis- and trans-isomers of straight chain C.sub.4, C.sub.5 and C.sub.6 olefins. U.S. Pat. No. 3,524,895 to Chen et al discloses the use of various aluminosilicates, particularly erionite, for a similar separation method.
Notwithstanding the above patents, however, no commercial adsorption process in the geometric isomers separation exists at the moment. The reason could be the reactivity between zeolites with olefinic compounds. In order to maintain the long term stability of the absorbent, an inert adsorbent is needed for the separation of geometric olefin isomers.
A new composition of matter referred to as "silicalite," has recently been developed and patented (see U.S. Pat. Nos. 4,061,724 and 4,104,294 to Gross et al). Silicalite is said by its inventors to have utility in separating organics from water.
I have made the discovery that silicalite is able to effect the separation of a trans-olefin from a cis-olefin with substantially complete elimination of the aforementioned undesired side effects of dimerization and polymerization.